The invention relates to an apparatus for the measurement of certain properties of particles held in a particle suspension, including a main measuring chamber divided into a first and a second chamber, wherein the first chamber includes a first electrode and is supplied with an electrolyte free from particles, which flows through a measurement aperture into the second chamber including a second electrode and within which the prevailing pressure is lower than the prevailing pressure in the first chamber, and where the particle suspension flows into the liquid stream ahead of the measuring opening whenever the pressure in the supply capillary tube which is disposed in the first chamber ahead of the measuring aperture is greater than the pressure prevailing in the first chamber.
Apparatus of this type is known (Spielmann/ Goren, Journal of Colloid and Interface Science 26, Pages 175-182 (1968); DAS No. 2 013 799). The mesurement described there is made according to the so-called "Coulter" Process (U.S. Pat. No. 2 656,508), which is based on the principle that, when a particle passes through the measuring aperture between the two electrodes, the different electrical properties of the particles on the one hand, and that of the electrolyte on the other hand, permit conclusions as to the properties of the particle which passed through the aperture (for example the volume) and, at the same time, when several particles pass the measuring aperture sequentially, the totality of all the pulses or signals which occur could be utilized, for example by classification, so as to obtain, for example, the volume distribution function. The known apparatus provides that the supply capillary dips into the first chamber, which is open to the atmosphere, ahead of the measuring aperture and that the particle suspension flows into the streaming electrolyte ahead of the measuring aperture due to the excess pressure prevailing in the supply capillary which is obtained in that the level of the particle suspension in the supply capillary is higher than the level of th electrolyte in the first chamber by an amount chosen so that the excess pressure in the supply capillary is sufficient to achieve the outward flow of the particle suspension in opposition to the flow resistances within the supply capillary tube.
By contrast to the apparatus described above, other devices (for example, those described in DAS No. 1 806 512, and in Thom: "Comparative investigations for electronic cell volume analysis", AEG-Telefunken, 1972, pages 16/17) which also operate on the above-mentioned "Coulter" process, but which provide that the outlet aperture of the supply capillary lies so close to the measuring aperture that, when the stream flows around the outlet aperture, the hydrodynamic suction, which may be calculated from Bernoulli's equation, is sufficient to draw the particle suspension out of the supply capillary without the necessity of providing excess pressure within the capillary relative to the static pressure in the chamber ahead of the measuring aperture.
A disadvantage of the known apparatus lies in that the outlet stream velocity of the particle suspension, i.e. the velocity with which it leaves the outlet aperture of the supply capillary, can be adjusted only with great difficulty and in a time-consuming manner, by changing the level of either the particle-free liquid or of the fluid in the suspension container. However, a simpler adjustment is desired, depending on the character of the particle suspension which must be examined, so as to account for different particle densities within the suspension, different particle sizes and different conditions of use of the data. For example, it may happen that a particular outlet velocity, which may be completely suitable for a particular particle suspension, would lead to an undesirable simultaneous passage of several particles with another particle suspension, or conversely, that the temporal separation between individual particles passing through the aperture becomes so large that an increase of the outlet velocity may be desirable for economic reasons.